In a conventional polarization measurement with the aid of a polarimeter, a measuring beam of defined wavelength and a defined polarization state is generated with the aid of a light source and with a polarizer and the sample to be investigated is irradiated with this polarized measuring beam. If an optically active substance, for example in dissolved form, is located in the sample, then the polarization state of the measuring beam changes during the irradiation of the sample. The polarization state of the measuring beam radiated through the sample is rotated in particular with regards to a polarization direction and is checked or determined by means of an evaluation unit. In this case, an analyzer is arranged within a beam path which likewise changes the polarization state of the measuring beam or only allows a certain polarization state to pass.
Either the orientation of the polarizer, the orientation of the analyzer or the orientations of both the polarizer and the analyzer is/are changed in order to minimize an intensity of measuring radiation received at a detector. From the orientations or rotations of the polarizer and/or of the analyzer, the change of the polarization state due to the irradiation through the sample can be deduced. In particular, the rotation of the polarization direction due to the irradiation through the sample can be determined. In turn, a concentration of optically active substances in the sample can, for example, be determined from the rotation value.
When carrying out a polarization measurement by using a conventional polarimeter, the measurement can be distorted, in particular by inhomogeneities of the sample, which can make the determination of the isotropic polarization properties of the sample difficult or impossible.
The following phenomena can inter alia fall within the concept of inhomogeneity:                1. gas bubbles which were either present before the filling of a sample container, particularly a cuvette, or which arise due to turbulences when filling the cuvette        2. particles made up of impurities        3. insufficient mixing of a sample. This can e.g. take place if the sample is prepared, e.g. diluted, prior to the measurement process and is then not mixed or homogenized well enough. A further example are solid samples which are dissolved for measurement and may contain particles of non-dissolved substance.        4. displaced residues of earlier samples.        
Inhomogeneities constitute a significant practical problem. The known methods for avoiding inhomogeneities are not reliable enough or are too complex or have another negative side effect, so that as before, there is a risk of false measurements due to inhomogeneous samples.
As inhomogeneities therefore cannot be reliably prevented, the need of being able to detect the same still exists. After the detection of inhomogeneities, countermeasures can then be taken, or at least false measurement values can be discarded. In order to be available to fulfill the, particularly in the pharmaceutical field, high quality requirements, the detection of inhomogeneities must be very reliable.
A known method for detecting inhomogeneities consists in directly visually examining the filled cuvette and after that to evaluate the same. However, carrying out this method is in many cases not possible, e.g. because the cuvette body consists of a non-transparent material or because the cuvette is inserted in the measuring device in such a manner that a direct look into the liquid is blocked.
Thus, in practice, the cuvette is often removed from the measuring device before each measurement, in order to be able to see directly through the cuvette. Although this procedure is informative, it is so impractical and time-consuming that it is very often omitted in practice. In addition, it prevents an automation e.g. by means of through-flow cuvettes, funnel cuvettes or autosamplers.
Furthermore, when carrying out these known methods, no objectively verifiable result can be documented, rather only the result of the subjective assessment of a user. This is often not sufficient for quality assurance.
U.S. Pat. No. 6,643,021 B1 discloses a checking method of a measuring system for determining an optical property, the optical property of a liquid sample being measured in that first light is projected in order to analyze the transmitted light. The method further comprises projecting additional light in a path of the first light or in a periphery thereof, in order to, based on an intensity of transmitted light of the second light, detect the presence or the absence of bubbles and/or particles which can interfere with the transmission of the first light. Following detection of the bubbles and/or particles, the same can be removed.
This approach can only succeed if it is ensured that all of the samples to be measured—in the case of homogeneous filling of the cuvette—have the same transmission for the second light beam. In an example of the U.S. Pat. No. 6,643,021 B1, the UV absorption of protein samples is e.g. measured with the first light beam in the measuring device. As proteins do not exhibit any absorption in the NIR, in this case, the intensity of the second light beam in the NIR can be used for detecting inhomogeneities.
The fundamental weakness of this known method, however, is the fact that an attenuation of the second light beam due to an absorption of a homogeneous sample is interpreted as an inhomogeneity. Thus, the method always fails when samples with an arbitrary absorption spectrum are to be measured. It is only possible in certain cases to find a suitable wavelength at which the investigated samples reliably do not exhibit any absorption.
A further disadvantage of the method from U.S. Pat. No. 6,643,021 consists in the fact that, even if it leads to a detection of an inhomogeneity, the intensity attenuation does not enable any differentiation of various types of inhomogeneities, such as bubbles or streaks due to sample carryover. This makes the determination of suitable countermeasures difficult. Sample carryovers or dirt particles can e.g. not be removed with the measures recommended in the patent for eliminating bubbles—such as shaking or ultrasound—rather the same can only be overcome by means of refilling the cuvette.
There is therefore a need for a device and a method for detecting inhomogeneities, in which the cuvette can remain in the measuring device in a position ready for measuring and which allow satisfactory documentation for a modern quality assurance.